If we look up at the sky on a clear night, we will see it full of stars. With the naked eye we can only see a few thousand of them, but with their telescopes astronomers count them by the millions. In our galaxy alone there are between 200,000 and 400,000 million suns, of different colors and sizes. But beyond the Milky Way, billions of other galaxies exist, each with a similar number of individual stars. Furthermore, these galaxies tend to travel in groups, which come together in increasingly larger super groups, until they give rise to enormous clusters made up of many thousands of individual galaxies… On our small scale, then, the Universe seems totally full of matter, organized into larger and larger structures. But that is a misleading image. Only by expanding the perspective even further will we be able to distinguish how matter is actually distributed on a large scale. Zoom out until the individual galaxies are just tiny points of light, and then we can see how they all line up into long filaments that converge at the points, the largest clusters, where the density, and gravity, is greatest. It is what we call ‘the cosmic web’, countless ‘rivers of light’ that intersect and connect as the strands do in a true spider’s web. And just like spider webs, on both sides of each strand, and up to the next strand, there are huge gaps in which there is…nothing. Vast regions of space, between 20 and 200 million light-years across, in which the vacuum is almost total. Gigantic cosmic deserts that extend from one end of the Cosmos to the other, that defy our understanding and that constitute, now, the greater part of the volume of the Universe in which we live. Those voids are, in fact, the largest objects we can observe. And now, a team of researchers led by Iranian physicists believe those incomprehensibly large voids could be tearing the entire Universe apart. How? Generating dark energy, the supposed responsible for the Universe expanding faster and faster. Their work, accepted for publication in the Monthly Notices of the Royal Astronomical Society, is now available on the ArXiv preprint server. The existence of cosmic voids has been known since the 1970s, but until recently they did not attract the attention of astronomers, who are more inclined to study ‘shining matter’, such as stars or galaxies. But twenty years later, in the 1990s, there was a discovery destined to change everything: the Universe continues to expand today, and not only that, but it does so faster and faster. That is, the speed at which it expands increases with each passing second. This is what is known as the ‘accelerated expansion’ of the Universe, and it began about 5,000 million years ago without anyone being able to explain why. That’s why the term ‘dark energy’ came about, a mysterious force that suddenly got going and dilates space itself, causing the objects in it to be further and further apart from each other. Despite this, the effects of the accelerated expansion are not felt within star systems or galaxies; there, the gravitational pull of matter is strong enough to cancel out the overall expansion. That’s why neither our Solar System nor the Milky Way is getting bigger due to dark energy. But voids, in which there is practically nothing, feel the effects of dark energy much more intensely. And if we want to find out what dark energy is, it makes perfect sense to investigate where its effects are strongest. In the new study, the researchers claim that dark energy is not only found in voids, but is caused by them. But how could this be possible? According to the authors, it is not enough just to verify the existence of these gaps, but also to study their dynamics. Like all structures in the Universe, cosmic voids were born much smaller and grew over time until they reached their current gigantic dimensions. Billions of years ago, all the matter in the Universe was fairly evenly distributed, so that initially there were no large differences in density from one place to another. But over time, any place that had just a little more matter than average began to attract more matter to it. And with more matter, its gravitational attraction also increased, which attracted even more matter and fueled its growth. Over billions of years, matter accumulated in this way to form galaxies, galactic groups and clusters. Of course, as these structures grew, the spaces between them ’emptied out’ and also became larger and larger. We can see the growth of these voids not as a passive process, but as a pressure of the voids themselves on the structures that surround them. For example, as voids grow larger, the walls of galaxies between them steadily thin out until they eventually dissolve, allowing the voids to merge. Over the next few billion years, the voids will end up completely dissolving the cosmic web, forcing all matter into isolated clumps separated by hundreds of millions of light-years of expanding void, the researchers say. In addition, the pressure exerted by the growing voids distorts space-time, just as any other source of matter or energy in the Universe would. And that space-time distortion means that as the voids expand, they push the galaxies that are on their edges, causing them to separate despite the gravitational attraction that exists between them. Lastly, the authors found that the cumulative effects of all the large voids in the Universe working together to dissolve the cosmic web lead to accelerated expansion. Which matches current estimates for dark energy. MORE INFORMATION Without eggs, sperm or uterus: the revolution in reproduction arrives A second meteorite could ‘finish off’ the dinosaurs Of course, more studies are needed to prove this idea. For one, more voids will need to be measured in order to better calculate their combined pressure. In addition, more information would also be needed about dark energy itself, and whether or not its strength has changed over the last few billion years. Still, the idea is intriguing: Dark energy may not be caused by some exotic force or process in the Universe, but simply a byproduct of the logical evolution of the vacuum.
Giant cosmic voids could be ripping the Universe apart